Reload hook (#1445)

* Add reload directive

* gofmt

* Fix default jitter and error message

* remove unneeded call to NextArg, add a couple negative setup tests

* Review feedback

plugin/reload/reload.go

@@ -0,0 +1,65 @@
+package reload
+
+import (
+	"crypto/md5"
+	"log"
+	"time"
+
+	"github.com/mholt/caddy"
+)
+
+// reload periodically checks if the Corefile has changed, and reloads if so
+
+type reload struct {
+	instance *caddy.Instance
+	interval time.Duration
+	sum      [md5.Size]byte
+	stopped  bool
+	quit     chan bool
+}
+
+func hook(event caddy.EventName, info interface{}) error {
+	if event != caddy.InstanceStartupEvent {
+		return nil
+	}
+
+	// if reload is removed from the Corefile, then the hook
+	// is still registered but setup is never called again
+	// so we need a flag to tell us not to reload
+	if r.stopped {
+		return nil
+	}
+
+	// this should be an instance. ok to panic if not
+	r.instance = info.(*caddy.Instance)
+	r.sum = md5.Sum(r.instance.Caddyfile().Body())
+
+	go func() {
+		tick := time.NewTicker(r.interval)
+
+		for {
+			select {
+			case <-tick.C:
+				corefile, err := caddy.LoadCaddyfile(r.instance.Caddyfile().ServerType())
+				if err != nil {
+					continue
+				}
+				s := md5.Sum(corefile.Body())
+				if s != r.sum {
+					_, err := r.instance.Restart(corefile)
+					if err != nil {
+						log.Printf("[ERROR] Corefile changed but reload failed: %s\n", err)
+						continue
+					}
+					// we are done, this hook gets called again with new instance
+					r.stopped = true
+					return
+				}
+			case <-r.quit:
+				return
+			}
+		}
+	}()
+
+	return nil
+}